This invention relates to step-tunable fiber lasers and, more specifically, to step-tunable fiber lasers for dense wavelength division multiplexed (DWDM) applications. It also pertains to methods by which such lasers may be made to operate.
A known method for increasing the capacity of a fiber optic communication system is wavelength division multiplexing, or WDM. WDM increases capacity by permitting the use of more than one optical carrier on a single fiber. WDM is accomplished by multiplexing the outputs of more than one laser onto the fiber. After transmission, at the receiving end, the multiple wavelengths are demultiplexed.
The concepts behind WDM have been extended to use a set of closely-spaced wavelengths in the 1550 nm window. The International Telecommunication Union (ITU) has proposed the use of a grouping or grid of wavelengths in this window. The channels are anchored to a reference at 193.10 THz and equally spaced in frequency, the closely spaced grids having channels 100 GHz or 50 GHz apart. In the wavelength range between 1528.77 nm and 1560.61 nm, the ITU 100 GHz grid comprises 41 channels. This method of WDM is known as dense wavelength division multiplexing, or DWDM.
Communication systems to implement this scheme thus must have laser sources which have emissions at each of the grid wavelengths. This could be accomplished by having a multitude of different laser sources, each having an emission wavelength corresponding to a respective one of the grid wavelengths. It would be far more convenient, cost-effective, and efficient, however, to have the capability of producing different wavelengths without having to correspondingly increase the number of different laser sources. A single laser source capable of having an emission wavelength set or tuned to any of the ITU grid wavelengths is therefore desirable. This type of laser source would also prove advantageous in sparing and hot sparing configurations for DWDM, and would allow for reconfigurable DWDM optical communication networks and network elements. For DWDM applications the laser source need not be continuously tunable, being that a step-tunable laser source would offer superior performance while, in general, be of simpler implementation. Such an apparatus should be tunable across the widest possible wavelength range, preferably across the whole ITU grid, and provide high optical output power across its whole wavelength operating range.
These and other desirable ends are met in the present invention through provision of an apparatus comprising a pump module for generating pump radiation, a ring laser resonator, an input coupler for optically coupling the pump radiation into the laser resonator, and an output coupler for optically coupling laser radiation out of the laser resonator. The laser resonator itself preferably includes an optically-coupled arrangement of a gain module which is pumped by the pump radiation, the pump radiation exciting the gain module thereby achieving lasing action in the laser resonator, a periodic filter, and a tunable filter.
The gain module, the periodic filter and the tunable filter may be optically coupled using single-mode optical fiber or polarization maintaining single-mode optical fiber.
The gain module preferably comprises an optical amplifier, which may be an erbium-doped fiber amplifier. The periodic filter may be a transmission filter which may be a fiber or fiber-coupled Fabry-Pxc3xa9rot micro-etalon filter. The periodic filter may alternatively be a reflection filter, which may be a sampled fiber Bragg grating or a set of sampled fiber Bragg gratings. Either way, the periodic filter preferably has spectral characteristics defined according to the ITU grid wavelengths. The periodic filter may be tunable, either electrically or mechanically tunable.
The tunable filter is preferably electrically tunable. It may comprise a fiber tunable filter or a fiber-coupled tunable filter, or, more specifically, a fiber Fabry-Pxc3xa9rot filter or a fiber-coupled Fabry-Pxc3xa9rot filter.
The laser resonator may also include means for ensuring unidirectional laser oscillation. The means for ensuring unidirectional laser oscillation may be an optical isolator positioned at one end of the gain module. Also, the gain module may include means for preventing reflections from other resonator components from disturbing laser emission, which may be a second optical isolator positioned at the opposite end of the gain module relative to the optical isolator used for ensuring unidirectional laser oscillation.
The resonator may also include a wavelength dependent filter having a spectral loss characteristic complementary to and compensating for the gain profile of the gain module so that a net gain of the gain profile and the loss characteristic is approximately constant across the laser operating wavelength range.
The resonator may also include means for preventing inter-etalon interactions between the periodic filter and the tunable filter. These means may be an optical isolator. The resonator may also include a polarization controller and polarizer.
The input coupler preferably comprises a fiber-fused wavelength division multiplexer and the output coupler preferably comprises a fiber-fused coupler. The pump module preferably comprises at least one pump laser diode or multiple pump laser diodes arranged according to a pump redundancy scheme.
The apparatus may also include means for locking of the periodic filter spectral response, which may be a wavelength reference control module. The wavelength reference control module may be made up of a fiber-fused coupler, a photodetector, and a wavelength reference filter. The wavelength reference filter may be a temperature-compensated fiber Bragg grating.
The apparatus may also include means for definition and stabilization of laser oscillation wavelength, which may be a laser wavelength control module. The laser wavelength control module may be made up of a fiber-fused coupler and a photodetector.
The apparatus may also include means for definition and stabilization of laser output power, which may be a laser output power control module. The laser output power control module may be made up of a fiber-fused coupler, a calibrated photodetector and an electrically variable optical attenuator.
The invention resides in part in the overall apparatus and in a resonator per se made up of a gain module, a periodic filter optically coupled to the gain module, and an electrically tunable filter optically coupled to the periodic filter.
The invention is also embodied in a method of generating a laser output at any one of a set of discrete frequencies, the method comprising the steps of providing pump energy to a gain medium in a laser cavity to excite a laser resonance, filtering the laser resonance using a periodic filter to limit the possible frequencies to the frequencies in the set, filtering the laser resonance using a tunable filter to limit the possible frequencies to one of the frequencies in the set, and tuning the tunable filter to select one of the frequencies in the set as an output frequency.
The invention is further embodied in a method of locking a spectral response of a periodic filter, the method comprising the steps of providing a fraction of radiation in a laser cavity to illuminate a stabilized wavelength reference filter, monitoring the stabilized wavelength reference filter response with a photodetector, controlling the spectral response of the periodic filter by means of a loop control unit, and operating the loop control unit in a closed-loop operation.
The invention is additionally embodied in a method of defining and stabilizing an output wavelength of a laser, the method comprising the steps of providing a fraction of the radiation in a laser cavity to illuminate a photodetector, controlling the spectral response of the tunable filter by means of a loop control unit, and operating the loop control unit in a closed-loop operation.
Also, the invention is additionally embodied in a method of defining and stabilizing the output power of a laser, the method comprising the steps of providing a fraction of the radiation in a laser output to illuminate a calibrated photodetector, controlling a variable optical attenuator insertion loss by means of a loop control unit, and operating the loop control unit in a closed-loop operation.